چ Materials of Sci. & Technology

COOLING CURVES

# BY - ABHIJIT PANCHMATIYA

- A cooling curve shows how the

temperature of a material (in this case, a

pure metal) decreases with time [Figure

(a) and (b)].

-The liquid is poured into a mold at the

pouring temperature, point A.

-The difference between the pouring

temperature and the freezing temperature

is the superheat.

-The specific heat is extracted by the

mold until the liquid reaches the freez-

ing temperature (point B).

-If the liquid is not well-inoculated, it

must be undercooled

Figure - (a) SHOWS:

Cooling curve for a pure metal that has not been well-

inoculated. The liquid cools as specific heat is removed (between points

A and B ). Undercooling is thus necessary (between points B and C ).

As the nucleation begins (point C ), latent heat of fusion is released

causing an increase in the temperature of the liquid. This process is

known as recalescence (point C to point D ). The metal continues to

solidify at a constant temperature ( T melting). At point E ,

solidification is complete. The solid casting continues to cool from this

point.

Figure - (b) SHOWS:

Cooling curve for a well-inoculated, but otherwise pure, metal.

No undercooling is needed. Recalescence is not observed.

Solidification begins at the melting temperature.

- As nucleation begins (point C), latent heat of fusion is given off, and the temperature rises.

-This increase in temperature of the undercooled liquid as a result of nucleation is known as

recalescence (point C to D).

-Solidification proceeds isothermally at the melting temperature (point D to E) as the latent heat given

off from continued solidification is balanced by the heat lost by cooling.

- This region between points D and E, where the temperature is constant, is known as the thermal

arrest.

- A thermal arrest, is produced because the evolution of the latent heat of fusion balances the heat being

lost because of cooling.

-At point E, solidification is complete, and the solid casting cools from point E to room temperature.

- If the liquid is well-inoculated, the extent of undercooling and recalescence is usually very small and

can be observed in cooling curves only by very careful measurements.

-If effective heterogeneous nuclei are present in the liquid, solidification begins at the freezing

temperature [Figure (b)].

The latent heat keeps the remaining liquid at the freezing tem- perature until all of the

liquid has solidified and no more heat can be evolved.

-Growth under these conditions is planar.

- The total solidification time of the casting is the time required to remove both the

specific heat of the liquid and the latent heat of fusion.

-Measured from the time of pouring until solidification is complete, this time is given

by Chvorinov’s rule.

- The local solidification time is the time required to remove only the latent heat of

fusion at a par-ticular location in the casting; it is measured from when solidification

begins until solidification is completed.

-The local solidification times (and the total solidification times) for liquids solidified

via undercooled and inoculated liquids will be slightly different.